The present invention relates to an optical disk drive for recording and reproducing data optically and, more particularly, to a track access control device applicable to an optical disk drive for moving a light beam over the consecutive tracks of a disk to a desired track and positioning it at the desired track.
With an optical disk drive for writing or reading data optically in or out of a medium or disk, it is necessary to move an optical head, i.e., a beam issuing from the head to desired one of a number of tracks formed on the medium in concentric circles or in a spiral and position it at the desired track. Since the head traverses a plurality of tracks intervening between it and the desired tack or target track, it is a common practice to control the movement of the beam by counting the tracks which the beam traverses. Specifically, the number of tracks which the beam traversed is subtracted from the distance (number of tracks) to the target track to generate a position signal. The beam is moved in a direction for reducing the position signal up to the target track. While the beam is in movement, a reference speed signal is generated which changes with a change in the distance to the target track. The moving speed of the beam is so controlled as to conform to the reference speed signal and thereby moved at high speed.
The above-described type of control is commonly effected not only with an optical disk drive but also with a magnetic disk drive. However, regarding an optical disk drive, the beam cannot be accurately stopped at the target track unless the speed control is provided with a high control frequency band (control loop gain) since the width of a track of an optical disk is far narrower than that of a magnetic disk.
The problem with a system of the type directly driving the head which is heavy is that it is difficult to set a high control frequency band, preventing the beam from being stopped at the target track stably and accurately. To eliminates this problem, a system which moves the beam by driving a lens adapted to converge the beam has been proposed, as disclosed in, for example, Japanese Patent Laid-Open Publication No. 131646/1985. In this kind of system, the lens has its speed controlled by the reference speed signal, i.e., the head is driven to follow the movement of the lens on the basis of a signal representative of a deviation of the lens from the head. Moving the small and light lens in place of the heavy head is successful in promoting speed control having sharp response and high control frequency band. However, the response of the mechanism for driving the head is too slow to sufficiently follow the rapid movement of the lens, increasing the positional deviation of the lens and head. The increase in the deviation of the lens and head means a noticeable deviation of the center of the lens from the axis of the beam to be converged and, therefore, critically aggravates the converging ability of the lens. Moreover, the deviation of the lens causes an track error signal representative of a deviation of the beam from the track to become offset and decrease in level. As a result, the control over the position of the beam and the speed control both of which are based on the track error signal are not stable.
In the light of the above, Japanese Patent Application No. 280769/1988 teaches an implementation for limiting a variation of the reference speed signal, which provides the moving speed of the lens, with respect to time (corresponding to an acceleration of the lens). This approach, however, complicates the circuit arrangement and, since the acceleration is limited, delays the arrival of the beam at the target track.